JavaScript triggers are the most useful feature for using Hotjar HeatMaps in one page applications, because without that you can't create HeatMaps on dynamic content or modals, dropdowns, etc.
So i will show you how i made it work on an Angular project.
I'm making the assumption that you already have an Hotjar account and initialize a project.

Include Hotjar in your project

First we need to include the tracking code in our index.html

<!-- Hotjar Tracking Code for https://yourpage.com/ -->
<script>
   (function(h,o,t,j,a,r){
       h.hj = h.hj || function(){ ( h.hj.q = h.hj.q || []).push(arguments) };
       h._hjSettings = { hjid:xxxxxxx, hjsv:x };
       a = o.getElementsByTagName('head')[0];
       r = o.createElement('script'); r.async = 1;
       r.src = t + h._hjSettings.hjid + j + h._hjSettings.hjsv;
       a.appendChild(r);
   })(window,document,'https://static.hotjar.com/c/hotjar-','.js?sv=');
</script>

With that we already have the hj function on our window object.

Hotjar Service

Now my goal was to make the access to the hj function reliable and mantainable across the Angular aplication, so i made a service:

@Injectable()
export class HotjarService {
  public getInstance() {
    return window['hj'] = window['hj'] || this.getHotjarQueue(); 
  }

  public trigger(eventName: string) {
    try {
      this.getInstance()('trigger', eventName);
    } catch (err) {
      console.error(err);
    }
  }

  public vpv(url: string) {
    try {
      this.getInstance()('vpv', url);
    } catch (err) {
      console.error(err);
    }
  }

  private getHotjarQueue() {
    return function () {
      (window['hj']['q'] = window['hj']['q'] || []).push(arguments);
    };
  }
}

Here we have the trigger function that allows us to register a new HeatMap, and the vpv function that register a new virtual page. The getInstance and the getHotjarQueue expose the window.hj if available to the service.

Conclusion

Making this little wrapper to the windows.hj we can make the use of Hotjar a lot more readable and mantainable in a big project. Using the function directly from the window object in frameworks as Angular is dangerous because of side effects and untraceable errors.